High-power microwave electron tubes, such as klystrons operating in the L-band, i.e. in a frequency band from 1 to 2 GHz, comprise at least one high-power RF output intended to be connected to a waveguide in order to transmit the RF power delivered by the tube to a microwave use circuit.
The output cavity of the klystron is connected to the waveguide via a microwave window transparent to the electromagnetic waves. The microwave window isolates the inside of the tube which is under vacuum from the outside, which is possibly under pressure of a gas. Furthermore, a transition after the microwave window is necessary for connecting the output of the window, which is of circular cylindrical shape, to the waveguide of rectangular cross section.
Conventional klystrons are amplifiers essentially having a microwave structure intended to amplify a microwave applied to an RF input of the tube. Amplification takes place by converting the kinetic energy of one or more electron beams passing through said structure into electromagnetic energy.
FIG. 1 shows a simplified diagram of a single-beam klystron of the prior art, which includes a vacuum chamber 10 along a longitudinal axis XX′, the klystron having:                an electron gun 20 with a biased cathode 22 emitting a continuous or pulsed electron beam 24, the electrons being accelerated by an anode 26, along the XX′ axis;        an input resonant cavity C1 for modulating the electron density of the beam when a radiofrequency (RF) signal is injected thereinto via an RF input 28 of the tube;        a circuit 30 for focusing (or confining) the electron beam along the XX′ axis, produced either from permanent magnets or by solenoids 32;        an electron collector 40, for collecting the electrons 44 of the electron beam, making it possible to receive most of the electrons emitted by the cathode 22 of the tube and to dissipate their residual kinetic energy after they are passed through the microwave structure; and        a resonant circuit 50, located between the anode 26 and the electron collector 40, having the function of converting some of the kinetic energy of the electrons in the beam in the resonant circuit to RF energy at the resonant frequency of the circuit. The resonant circuit 50 is also termed an interaction structure. The resonant circuit forms an integral part of the klystron and comprises a series of resonant cavities C1, C2, C3, C4, in the microwave structure.        
The klystron further includes at least one RF power output 58 in the form of a waveguide connected to an output transition 60 (FIG. 1) for extracting, to the outside of the tube, the amplified RF power along an axis ZZ′.
The output transition 60, in this embodiment of a klystron, includes a ceramic window 61 for sealing between the inside of the tube under vacuum and an application waveguide (not shown in FIG. 1) under pressure of a gas and the connection by means of a flange 64 to the waveguide.
In the case of a klystron with a high output power, the waveguide used is, for example in the L-band, a WR650 waveguide of rectangular cross section.
FIG. 2 shows a partial view (delimited by dashed lines B) of an output transition of the prior art of the klystron shown in FIG. 1.
The output transition 60 includes, on the output side of the klystron, via a separating window (not shown in FIG. 2), a cylindrical portion 66 of circular cross section connected to the output of the klystron and, on the opposite side, another portion 68 in the form of a tube of rectangular cross section that includes the flange 64 for connection to the application waveguide. Inside the transition, the cylindrical surface and the plane surfaces of the rectangular section are joined by a fillet 70 of radius r.
The microwave output circuit is exposed to intense electric fields through the passage of the RF power output by the high-power klystron and notably in the output transition 60. If the electric field generated by the RF power wave in the output transition produces, between certain points (or electrodes) of the transition, a voltage above the breakdown voltage of said transition, then an electric arc is produced, which may degrade the klystron.
The output of the klystron includes arc detectors, but despite having fast response times, these detectors can only be triggered at the moment when the breakdown occurs, which nevertheless limits the behavior of the arc over time.
The breakdown resistance of the power output circuit of the klystron depends on various parameters, such as the standing wave ratio in the output circuit, the harmonics of the RF signal, the temperature of the transition and of the waveguide, as well as the influence of other elements present in the output circuit, such as measurement components or couplers, but also particles suspended in the gas of the pressurized circuit which reduce the arc initiation resistance.
To avoid arcs being initiated in the RF output circuit of the tube, it is important for the critical breakdown threshold not to be exceeded. The electric field in the output circuit corresponding to this limit is what is called the electrical strength or breakdown strength of the gas. This breakdown strength depends on the gas pressure multiplied by the distance between those parts of the output circuit liable to generate the arc (equivalent to electrodes) and on the material of those parts of the circuit.
In general, the arcs are produced notably in the transition connecting the output of the tube to the waveguide inside the transition, at the fillet 70 between the cylindrical portion 66 and the rectangular portion 68 of the transition. One weakness of this type of transition of the prior art shown in FIG. 2 is that of having a region with an electric field locally increased because of the geometry of this zone, the electric field being able to locally exceed the breakdown threshold. The electromagnetic field in the transition reaches a maximum level at the fillet 70 in a central portion of this fillet.
The electric arc resistance of this type of transition of the prior art is limited because of a small fillet radius r, for example around 2 to 3 mm in the L-band, between the internal surface of the cylindrical portion 66 and that of the rectangular portion 68.
The breakdown threshold in a closed volume, through which an electromagnetic wave travels, depends on the composition of the gas used but also on the pressure. In the case of dry air, a maximum electric field of the order of 1 kV/mm is usual in the RF power circuit.
For example, under specified power and frequency conditions, the electric field in the transition may locally be twice the electric field in the waveguide. As a result, the transition becomes the element of the microwave circuit that limits the RF power that can be transmitted, because of the risk of breakdown.
To increase the breakdown threshold of this type of transition of the prior art, one solution consists in using a gas such as sulphur hexafluoride or SF6, under pressure in the transition and in the application waveguide.
The microwave transition 60 and the application waveguide must be pressurized with SF6 or an equivalent type gas. Pressurization with this type of SF6 gas makes it possible to transmit a peak output power from the klystron that is much higher than for pressurization with air or nitrogen.
However, the use of SF6 gas for the pressurization has drawbacks. Specifically, SF6 is a greenhouse gas and maintenance of the output circuit, either to pressurize it or to depressurize it, requires precautions to be taken in order to prevent the gas from escaping into the atmosphere.
A significant procedure must be followed so as to prevent this gas from leaking into the atmosphere, such as the use of bottles for collecting the gas in the output circuit of the klystron and the draining of the circuit by pressurizing the circuit with another gas that does not have these drawbacks.
Furthermore, the gas SF6, although harmless to personnel when it is pure, may subsequently become harmful when it is being replaced after use in the output circuit. This is because repetitive breakdowns in the RF output circuit produce, owing to the initially pure SF6 gas decomposing, other gases which are themselves harmful.